This invention relates to compounds that are useful as solvents for the electrolyte salts used in electrochemical devices. This invention further relates to electrolyte compositions comprising at least one such compound and at least one such salt. In other aspects, this invention also relates to electrochemical devices comprising the electrolyte compositions and to articles comprising the electrochemical devices.
The rapid development of electronic devices has increased market demand for electrochemical devices such as fuel cells, capacitors, electrochromic windows, and battery systems. In response to the demand for battery systems in particular, practical rechargeable lithium batteries have been actively researched. These systems are typically based on the use of lithium metal, lithiated carbon, or a lithium alloy as the negative electrode (anode).
Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells have consisted of a non-aqueous lithium ion-conducting electrolyte composition interposed between electrically-separated, spaced-apart positive and negative electrodes. The electrolyte composition is typically a liquid solution of lithium electrolyte salt in nonaqueous aprotic organic electrolyte solvent (often a solvent mixture).
The selection of electrolyte solvents for rechargeable lithium batteries is crucial for optimal battery performance and involves a variety of different factors. However, long-term stability, ionic conductivity, safety, and wetting capability tend to be the most important selection factors in high volume commercial applications.
Long-term stability requires that an electrolyte solvent be intrinsically stable over the battery""s range of operating temperatures and voltages and also that it be either unreactive with electrode materials or that it effectively form a passivating film with good ionic conductivity. Ionic conductivity requires an electrolyte solvent that effectively dissolves lithium electrolyte salts and facilitates lithium ion mobility. From the viewpoint of safety, the characteristics of low volatility, low flammability, low combustibility, and low toxicity are all highly desirable. It is also desirable that the battery""s electrodes and separator be quickly and thoroughly wetted by the electrolyte solvent, so as to facilitate rapid battery manufacturing and optimize battery performance.
Aprotic liquid organic compounds have been the most commonly used electrolyte solvents for lithium batteries. Often, compounds such as ethers or carbonic acid esters (carbonates) have been utilized, as these compounds typically share the desirable properties of oxidative stability at positive electrodes operating at less than about 4.4 V vs. Li+/Li, low reactivity with lithium-containing negative electrodes, and a thermodynamically favorable interaction with lithium ions (which is manifested in the electrolyte composition as a high degree of dissociation of the anion and the lithium cation of the electrolyte salt).
The most commonly used aprotic organic electrolyte solvents for use in lithium batteries include cyclic esters (for example, ethylene carbonate, propylene carbonate, xcex3-butyrolactone), linear esters, cyclic ethers (for example, 2-methyltetrahydrofuran, 1,3-dioxolane), linear ethers (for example, 1,2-dimethoxyethane), amides, and sulfoxides. A mixed solvent is sometimes preferred, since the properties of the electrolyte composition (conductance, viscosity, etc.) and its reactivity towards lithium can often be xe2x80x98tailoredxe2x80x99 to give optimum performance.
Less traditional solvents such as carboxylic acid esters, sulfoxides, sulfones, and sulfonamides have been used as electrolyte solvents with varying success. Sulfones are typically solids at room temperature. Sulfones such as tetramethylene sulfone (sulfolane) and ethyl methyl sulfone, however, have been used as electrolyte solvents. Dimethylsulfone has also been utilized, but, with a melting point of 107xc2x0 C., its utility has been limited to batteries that operate at elevated temperatures (that is, at temperatures above which the electrolyte composition can be maintained in the liquid state).
Drawbacks to the use of conventional lithium battery electrolyte solvents are generally related to their low boiling points and high flammabilities or combustibilities. Some solvents, such as the cyclic carbonates ethylene carbonate and propylene carbonate, have boiling points above 200xc2x0 C. However, many electrolyte solvents have boiling points that are substantially lower and have flash points less than 100xc2x0 F. Such volatile solvents can ignite during catastrophic failure of a fully or partially charged battery that has undergone, for example, a rapid discharge due to a short circuit. Additionally, volatile solvents present difficulties in the preparation and storage of electrolyte compositions as well as in addition of the composition to the battery during the manufacturing process. Another common problem of some conventional electrolyte solvents is that they often have a surface energy that is too high to spontaneously wet the battery components.
Thus, there remains a need in the art for electrolyte solvents that have reduced volatility, flammability, and combustibility (relative to conventional solvents), yet effectively dissolve electrolyte salts to form stable electrolyte compositions that adequately wet electrochemical device components and that exhibit adequate ionic conductivities over a range of operating temperatures.
Briefly, in one aspect, this invention provides novel carbonate compounds and novel carbamate compounds that are useful as electrolyte solvents for the electrolyte salts used in electrochemical devices. The compounds comprise at least one carbonate or carbamate moiety that is directly bonded only to groups selected from the group consisting of alkyl groups, cycloalkyl groups, alkenyl groups, cycloalkenyl groups, alkynyl groups, cycloalkynyl groups, and combinations thereof (for example, cycloalkyl-substituted alkyl groups), the groups optionally containing one or more catenary heteroatoms. (As used herein, the term xe2x80x9ccatenary heteroatomsxe2x80x9d means heteroatoms (for example, nitrogen, oxygen, or sulfur) that replace one or more carbon atoms of a group in a manner such that the heteroatom is bonded to at least two carbon atoms of the group.) At least one of the directly-bonded groups comprises at least one sulfonyl moiety.
Preferably, the compounds are sulfonyl-containing dialkyl carbonate compounds. More preferably, the compounds are sulfonyl-containing dialkyl carbonate compounds comprising lower alkyl groups that each contain no more than about six (most preferably, no more than about four) carbon atoms.
It has been discovered that the above-described novel compounds have surprisingly high boiling points and low volatilities and thus, in general, are less flammable and less combustible than conventional electrolyte solvents. Yet the compounds quite effectively dissolve electrolyte salts to provide electrolyte compositions that adequately wet electrochemical device components (such as separators) and that exhibit adequate ionic conductivities for use in electrochemical devices over a range of operating temperatures (for example, from about 20xc2x0 C. to about 80xc2x0 C. or even higher, depending upon the power requirements for a particular application). The compounds (and electrolyte compositions comprising the compounds) also present fewer difficulties in storage and handling than do conventional materials, due to their lower volatility, flammability, and combustibility.
The compounds are particularly well-suited for use in high-temperature batteries (batteries that are designed to function at temperatures above, for example, about 60xc2x0 C.). In such batteries, electrolyte compositions comprising the compounds exhibit adequate conductivities, while being less likely to ignite during catastrophic battery failure than conventional electrolyte compositions.
Thus, the novel compounds of the invention meet the need in the art for electrolyte solvents that have reduced volatility, flammability, and combustibility (relative to conventional solvents), yet effectively dissolve electrolyte salts to form stable electrolyte compositions that adequately wet electrochemical device components and that exhibit adequate ionic conductivities over a range of operating temperatures.
In other aspects, this invention also provides electrolyte compositions comprising (a) at least one compound of the invention, and (b) at least one electrolyte salt; electrochemical devices comprising the electrolyte compositions; and articles comprising the electrochemical devices.
Electrolyte Solvents
The novel compounds of the invention comprise at least one (preferably, only one) carbonate or carbamate moiety (preferably, carbonate) that is directly bonded only to alkyl, cycloalkyl, alkenyl, alkynyl, cycloalkynyl, or cycloalkenyl groups, or to combinations thereof (for example, cycloalkyl-containing alkyl groups, cycloalkyl-containing alkenyl groups, cycloalkenyl-containing alkyl groups, and cycloalkenyl-containing alkenyl groups, where the cyclic moiety can be either monovalent or divalent depending upon its location within the alkyl or alkenyl group). (Thus, the carbonate or carbamate moiety is directly bonded only to groups other than aryl groups or aryl-containing groups.) Preferably, the carbonate or carbamate moiety is bonded only to alkyl groups. At least one of the directly-bonded groups (preferably, only one) comprises at least one sulfonyl moiety (preferably, only one such moiety). Although the directly-bonded groups can optionally contain one or more catenary heteroatoms (for example, nitrogen, oxygen, or sulfur atoms), preferred compounds contain no catenary heteroatoms other than the sulfur atom of the sulfonyl moiety. Preferably, the directly-bonded groups each contain no more than about six carbon atoms (more preferably, no more than about four), so that the ratio of the number of oxygen atoms to the number of carbon atoms in the compound is in the range of about 0.4 to about 1 (more preferably, from about 0.66 to about 1; most preferably, from about 0.83 to about 1).
A preferred class of the novel compounds of the invention is that which can be represented by the following general formula (I):
H(CH2)mSO2(CH2)nQ(R)p xe2x80x83xe2x80x83(I) 
wherein Q is a carbonate moiety, xe2x80x94Oxe2x80x94C(O)xe2x80x94Oxe2x80x94, or a carbamate moiety, xe2x80x94Oxe2x80x94C(O)xe2x80x94Nxe2x80x94 (preferably, a carbonate moiety); each R is independently a group selected from the group consisting of alkyl groups, cycloalkyl groups, alkenyl groups, cycloalkenyl groups, alkynyl groups, cycloalkynyl groups, and combinations thereof (preferably, alkyl groups), the groups optionally containing one or more catenary heteroatoms; m is an integer of one to about four; n is an integer of one to about four; and p is an integer of 1 when Q is a carbonate moiety and is an integer of 2 when Q is a carbamate moiety. Preferably, R contains no catenary heteroatoms, R has no more than about six carbon atoms (more preferably, no more than about four carbon atoms), and the sum of m and n is less than or equal to about six (more preferably, less than or equal to about four). 
The compounds of the invention can be prepared by combining (preferably, in the presence of a base or hydrogen ion acceptor, and, optionally, in an organic solvent) (a) an alcohol comprising at least one sulfonyl moiety and (b) an alcohol-reactive compound (for example, a carbonic acid halide or a carbamic acid halide) that is capable of reacting with the alcohol to form a carbonate (a carbonic acid ester) or a carbamate (a carbamic acid ester) via a condensation reaction (with the elimination of, for example, a mineral acid). Reactions of this type are well-known in organic chemistry and have been described, for example, by Jerry March in Advanced Organic Chemistry, 4th Edition, p. 392, John Wiley and Sons, New York (1992).
Preferably, a base or hydrogen ion acceptor (more preferably, an organic base such as an amine) is present to neutralize any mineral acid that is generated during the condensation reaction. Examples of useful organic bases include heterocyclic aromatic amines (for example, pyridine) and alkylamines (for example, diisopropylamine and triethylamine). Those skilled in the art will recognize the types and limitations of organic bases that are useful for such condensation reactions.
Alcohols that can be used to prepare the compounds of the invention include primary, secondary, and tertiary alcohols (preferably, primary or secondary; more preferably, primary) that comprise at least one sulfonyl moiety, but that do not comprise interfering functional groups (for example, groups such as primary amine, thiol, or carboxylic acid groups, which can interfere with the reaction of the alcohol by reacting with the alcohol-reactive compound; or groups such as isocyanate, carboxylic acid, acyl halide, and ester groups, which can react with the hydroxyl group of the alcohol itself). Representative examples of suitable alcohols include: 
and the like, and mixtures thereof.
Compounds that are suitable for use as the alcohol-reactive compound comprise a hydroxyl-reactive functional group (for example, an acid halide-group) that is capable of reacting with the hydroxyl group of an alcohol in a condensation reaction to form a carbonate or a carbamate, with the elimination of a small molecule (for example, a mineral acid such as hydrochloric acid). The compounds preferably have one such hydroxyl-reactive functional group, although it is understood that the products of reactions of compounds with more than one such hydroxyl-reactive functional group are within the scope of this invention. The alcohol-reactive compounds do not comprise other functional groups (such as, for example, alcohol, amine, thiol, or carboxylic acid groups) that are capable of reacting with the hydroxyl-reactive functional group.
Suitable alcohol-reactive compounds include derivatives of formic acid (for example, alkyl haloformates, such as ethyl chloroformate and methyl chloroformate; alkenyl haloformates; cycloalkyl haloformates; and cycloalkenyl haloformates) and nitrogen-containing compounds that are isoelectronic with such formic acid derivatives (for example, alkyl halocarbamates, such as dimethyl chlorocarbamate; alkenyl halocarbamates; cycloalkyl halocarbamates; and cycloalkenyl halocarbamates). Representative examples of suitable alcohol-reactive compounds include: 
If desired, alcohol-reactive compounds that contain one or more sulfonyl moieties can be utilized. For example, a sulfonyl-containing alcohol can be allowed to react with phosgene to provide a sulfonyl-containing chloroformate, followed by the reaction of the sulfonyl-containing chloroformate with an alcohol. This two-step process can be carried out by using the same alcohol in both steps, or by using two different alcohols. This process is a preferred process for preparing symmetrical carbonate compounds comprising at least two sulfonyl moieties, as shown for example below: 
The two reactants (the alcohol and the alcohol-reactive compound) can be added to a reaction vessel in any order. The condensation reaction can be carried out at any of a range of temperatures, depending upon whether a sealed or unsealed reaction vessel is utilized. However, it is generally preferable to carry out the reaction at temperatures below the boiling point of the lower boiling reactant (for example, at temperatures between about xe2x88x9210xc2x0 C. and about 50xc2x0 C.; more preferably, between about 0xc2x0 C. and about 25xc2x0 C.) in an open vessel.
If desired, the reaction can be carried out in a relatively polar organic solvent (for example, a solvent such as tetrahydrofuran, glyme, chloroform, 1,2-dichloroethane, diethyl ether, tert-butyl methyl ether, and the like, and mixtures thereof). Preferably, no organic solvent is used, but the reaction is carried out in the presence of an excess of base. The desired product can be isolated by adding the resulting reaction mixture to water (or to aqueous mineral acid, if base has been utilized) and then extracting the product with a slightly polar solvent (for example, dichloromethane).
A preferred process for preparing the compounds of the invention comprises the steps of 1) combining an alcohol with an about 5% to about 100% molar excess of organic amine in a reaction vessel at about 0xc2x0 C.; 2) adding an alcohol-reactive compound to the vessel at a rate sufficient to maintain the temperature of the resulting reaction mixture at about 0xc2x0 C.; 3) allowing the reaction mixture to warm to room temperature; 4) adding the reaction mixture to aqueous mineral acid; 5) extracting the desired product with a solvent in which the product is soluble; 6) drying the resulting solution of the product with a conventional solid drying agent (for example, anhydrous sodium sulfate); and 7) removing the solvent by evaporation to isolate the product. The isolated product can then be purified, if desired, by conventional methods such as chromatography, distillation, crystallization, etc., all of which are well-known to those skilled in the art.
An alternative process for preparing the compounds of the invention involves transesterification, which is described, for example, by Jerry March in Advanced Organic Chemistry, 4th Edition, pp. 397-398, John Wiley and Sons, New York (1992). This process involves combining (a) an alcohol comprising at least one sulfonyl moiety, as described above (except less preferably in this case a tertiary alcohol) and (b) a carbonate compound (preferably, a symmetrical carbonate compound), with heating and in the presence of a catalyst such as a strong acid, a strong base, or a high-valency early transition metal complex (for example, Ti(IV) propoxide). This process can also be used to prepare symmetrical carbonate compounds comprising more than one sulfonyl moiety, as shown for example below: 
Electrolyte Compositions
The compounds of the invention can be utilized to prepare electrolyte compositions comprising (a) at least one compound of the invention; and (b) at least one electrolyte salt. Electrolyte salts that are suitable for use in preparing the electrolyte compositions of the invention include those salts that comprise at least one cation and at least one weakly coordinating anion, that are at least partially soluble in a selected compound of the invention (or in a blend thereof with one or more other compounds of the invention or one or more conventional electrolyte solvents), and that at least partially dissociate to form a conductive electrolyte composition. Preferably, the salts are stable over a range of operating voltages, are non-corrosive, and are thermally and hydrolytically stable.
Suitable cations include alkali metal, alkaline earth metal, Group IIB metal, Group IIIB metal, transition metal, rare earth metal, and ammonium (for example, tetraalkylammonium or trialkylammonium) cations, as well as a proton. Preferred cations for battery use include alkali metal and alkaline earth metal cations.
Suitable weakly coordinating anions include NO3xe2x88x92; Brxe2x88x92; Ixe2x88x92; BF4xe2x88x92; PF6xe2x88x92; AsF6xe2x88x92; ClO4xe2x88x92; SbF6xe2x88x92; HSO4xe2x88x92; H2PO4xe2x88x92; organic anions such as alkane, aryl, and alkaryl sulfonates; fluorinated and unfluorinated tetraarylborates; carboranes and halogen-, alkyl-, or haloalkyl-substituted carborane anions including metallocarborane anions; teflates (for example, xe2x88x92OTeF5, xe2x88x92B(OTeF5)4, and xe2x88x92Pd(OTeF5)4); and fluoroorganic anions such as perfluoroalkanesulfonates, cyanoperfluoroalkanesulfonylamides, bis(cyano)perfluoroalkanesulfonylmethides, bis(perfluoroalkanesulfonyl)imides, bis(perfluoroalkanesulfonyl)methides, and tris(perfluoroalkanesulfonyl)methides; and the like. Preferred anions for battery use include fluoroinorganic anions (for example, BF4xe2x88x92, PF6xe2x88x92, and AsF6xe2x88x92) and fluoroorganic anions (for example, perfluoroalkanesulfonates, bis(perfluoroalkanesulfonyl)imides, and tris(perfluoroalkanesulfonyl)methides).
The fluoroorganic anions can be either fully fluorinated, that is perfluorinated, or partially fluorinated (within the organic portion thereof). Preferably, the fluoroorganic anion is at least about 80 percent fluorinated (that is, at least about 80 percent of the carbon-bonded substituents of the anion are fluorine atoms). More preferably, the anion is perfluorinated (that is, fully fluorinated, where all of the carbon-bonded substituents are fluorine atoms). The anions, including the preferred perfluorinated anions, can contain one or more catenary heteroatoms such as, for example, nitrogen, oxygen, or sulfur.
Preferred fluoroorganic anions include perfluoroalkanesulfonates, bis(perfluoroalkanesulfonyl)imides, and tris(perfluoroalkanesulfonyl)methides. The perfluoroalkanesulfonates and bis(perfluoroalkanesulfonyl)imides are more preferred anions, with the perfluoroalkanesulfonates being most preferred.
Preferred salts for battery use are lithium salts. More preferred are lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(perfluoroethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium tris(trifluoromethanesulfonyl)methide or a mixture thereof.
The electrolyte compositions of the invention can be prepared by combining at least one compound of the invention and at least one electrolyte salt such that the salt is at least partially dissolved in the compound of the invention at the desired operating temperature. One or more conventional electrolyte solvents (for example, propylene carbonate or dimethoxyethane) or other conventional additives (for example, a surfactant) can also be present, if desired. The electrolyte salt is preferably employed at a concentration such that the conductivity of the electrolyte composition is at or near its maximum value (typically, for example, at a Li+ molar concentration of around 0.5-2.0 M, preferably about 1.0 M, for electrolytes for lithium batteries), although a wide range of other concentrations will also serve.
Electrochemical Devices
The electrolyte compositions of the invention can be used as electrolytes in electrochemical devices including, for example, such devices as fuel cells, batteries, capacitors, and electrochromic windows. Such devices typically comprise at least one first electrode, at least one second electrode, at least one separator, and an electrolyte composition of the invention.
The electrodes of, for example, a lithium battery generally consist of a metallic foil or particles of active material blended with a conductive diluent such as carbon black or graphite bound into a polymeric material binder. Typical binders include polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene (EPDM) terpolymer, and emulsified styrene-butadiene rubber (SBR), and the binder can be cross-linked. The binder can also be, for example, a solid carbon matrix formed from the thermal decomposition of an organic compound. The metallic foil or composite electrode material is generally applied to an expanded metal screen or metal foil (preferably aluminum, copper, nickel, or stainless steel) current collector using any of a variety of processes such as coating, casting, pressing or extrusion.
Some examples of suitable first electrodes are lithium metal, aluminum, lithium metal alloys, sodium metal, platinum and palladium and alloys thereof, carbon-based materials such as graphite, coke, carbon, pitch, transition metal oxides (for example, LiTi5O12 and LiWO2), and lithiated tin oxide. In the case of lithium ion batteries, the lithium can be intercalated into a host material such as carbon (that is, to give lithiated carbon) or carbon alloyed with other elements (such as silicon, boron or nitrogen), a conductive polymer, or an inorganic host that is intercalatable (such as LixTi5O12). The material comprising the first electrode can be carried on foil (for example, nickel and copper) backing or pressed into expanded metal screen and alloyed with various other metals.
Active second electrode materials generally provide device voltages of at least about 3.0 volts at a full state of charge relative to Li/Li+. Suitable second electrode materials include graphite; carbon; aluminum; MnO2; platinum, palladium, and alloys thereof; a composite oxide comprising Li and a transition metal such as LiCoO2, LiNiO2, LiV3O8, LiMn2O4, etc.; V2O5; V6O13; Ba2SmNiO5; SmMnO3; Sm3Fe5O12; EuFeO3; EuFe5O12; EuMnO3; LaNiO3; La2CoO4 and LaMnO3 (including the charged and discharged forms of these materials); oxides of ruthenium or tungsten; indium tin oxide; and conducting polymers such as polypyrrole, polysulfides and polyvinylferrocene. In primary batteries, the second electrode can be fluorinated carbon (for example, (CF)n), SO2Cl2, Ag2V4O11, Ag2CrO4, sulfur, polysulfide, or an O2 or SO2 electrode.
Lithium batteries generally contain a separator to prevent short-circuiting between the first and second electrodes. The separator often consists of a single-ply or multi-ply sheet of microporous polymer (typically polyolefin, for example, polyethylene, polypropylene, or combinations thereof) having a predetermined length and width and having a thickness of less than about 1.0 mil (0.025 mm). For example, see U.S. Pat. No. 3,351,495 (Larsen et al.), U.S. Pat. No. 4,539,256 (Shipman et al.), U.S. Pat. No. 4,731,304 (Lundquist et al.) and U.S. Pat. No. 5,565,281 (Yu et al.). The pore size in these microporous membranes, typically about 5 microns in diameter, is sufficiently large to allow transport of ions but is sufficiently small to prevent electrode contact, either directly or from particle penetration or dendrites which can form on the electrodes.
The electrochemical devices of the invention can be used in various electronic articles such as computers, power tools, automobiles, telecommunication devices, and the like.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. In the following examples, the structures of prepared compounds were determined using nuclear magnetic resonance spectroscopy, infrared spectroscopy, and mass spectrometry. Column chromatography was conducted on silica gel, using a mixture of one part by volume acetonitrile and 2 parts by volume chloroform.